This invention relates to a photometer tube for a microscope, having a prism which divides the ray path coming from the objective of the microscope between the eyepiece or ocular of the microscope and a photoelectric receiver, and also having a measurement diaphragm arranged in front of the photoelectric receiver, and means for reflecting an image of the measurement diaphragm back into the eyepiece.
Such photometer tubes are known, for instance, from U.S. Pat. No. 3,421,806 issued Jan. 14, 1969, Weber; Swiss Pat. No. 615,762 and its corresponding British Pat. No. 1,582,346 published Jan. 7, 1981, Leitz; and German Fed. Rep. Pat. No. 3,443,728 and its corresponding English language European patent application published June 4, 1986 (Bulletin 86/23) as publication 0,183,416 A2, Shindl. The means for reflecting the measurement diaphragm serves to make the object being viewed and the measurement diaphragm simultaneously visible to the viewer in the binocular tube. For this purpose, an image of the rear-illuminated measurement diaphragm is focused superimposed on the intermediate image of the object in the binocular tube.
In the apparatus mentioned above with reference to the state of the art, the focusing of the measurement field diaphragm into the binocular tube is effected in the manner that behind a second and generally unused exit of the divider prism there is mirror optics which produces a preferably upright and laterally correct image of the measurement field diaphragm on itself and into the intermediate image plane conjugated with the plane of the mesurement field diaphragm within the binocular tube. For this purpose, corner mirrors or plane mirrors with a focusing optical system in front thereof are used.
These known solutions have the disadvantage that they require a relatively large number of optical components. In addition, there is required a divider prism having two exits and space for arranging the components of the return-reflection means behind the second exit of the prism. But such space is frequently not available. It is therefore not readily possible, when using these known solutions, to re-equip already existing tubes as photometer tubes.